If you’re using the on site printing set up to raise money for charity, tell your guests about it. Also – don’t tuck the studio set up away in another room, where guests may miss it.
If your event has an MC, tell them to mention the photos are being taken. This will encourage your guests involvement.
You could tell your guests about the print on site event photography, on the invite to the event itself. In that way, if the images are being sold to raise run funds for a charity, people can ensure that they have a means of payment on them.